The present invention relates to flame scanners, and more specifically, to an apparatus and method for detecting the presence of a fault in the scanner sensor or connecting cable.
In utility boilers and other devices in which fossil fuels are burned, safety considerations dictate that elaborate fuel and air supply control systems be provided to ensure that once ignition has been achieved and a flame established, fuel and air are not admitted into the combustion chamber unless a flame is present therein. If fuel and air were admitted into a hot combustion chamber wherein the flame had expired, a disasterous explosion could result from accumulation of large quantities of unburnt fuel and air within the combustion chamber. The heart of such a safety system is the flame scanner, the purpose of which is to monitor the flame and provide a signal indicating the presence or absence of flame within the combustion chamber.
The most commonly used flame scanners of today monitor the electromagnetic radiation, i.e., light, produced during the combustion of fossil fuel. A light sensitive sensing device, such as a Geiger-Muller glow discharge tube or a photodiode, views the flame and in response to the varying intensity of the electromagnetic radiation it receives, produces as its output a varying current signal. This current signal is processed through well-known circuitry and an indication of the presence or absence of flame within the combustion chamber is generated.
It is extremely undesirable to have a flame scanner indicate the presence of a flame in the combustion chamber when in fact there is no such flame. A major problem with scanners of the type described above is that they have been known to wrongly indicate the presence of a flame long after the flame within the combustion chamber has expired. One reason for such an occurrence is that the photosensor malfunctions and continues to output a varying current signal even though no light is striking it.
The prior art solution to this problem has been to periodically or randomly activate mechanical means to block the monitoring view of the photosensor so that a flame-out condition is simulated. If the sensor still outputs a varying current signal indicating the presence of a flame within the combustion chamber while the mechanical blocking means is disposed so as to block the sensors view of the flame, the presence of a fault in the photosensor is indicated.
One type of mechanical blocking means well-known in the prior art is the shutter. For example, U.S. Pat. No. 3,538,332 discloses a flame scanner with a sensor head which incorporates a mechanically activated shutter which opens and closes in jaw-like fashion so as to periodically interupt the line of sight between the photosensor and the flame. A disadvantage of the mechanical shutter is that the shutter may stick in the open position thereby preventing a check from being made of the sensor integrity, or it may stick in the closed position thereby rendering the scanner inoperative.
Another type of mechanical blocking means known in the prior art is illustrated in U.S. Pat. No. 3,594,746. Described therein is a flame scanner fault detection system wherein a ball is periodically pneumatically operated to seat in the view port of the scanner head thereby interrupting the scanners view of the flame. When pneumatic activation ceases, the ball drops under the influence of gravity from the scanner view port. If the scanner indicates the presence of flame when the ball is pneumatically activated, the presence of a fault is indicated. A disadvantage of this device is that the ball can become stuck when it seats in the view port of the scanner thereby rendering the scanner inoperative.
Another problem associated with the use of mechanical blocking means in a fault detection system is that they operate either randomly or, at best, periodically rather than continuously. That is, the operation of the scanner must be interrupted in order for the system to be tested for a defect.
Thus, there is evidence in the prior art of a need for a scanner fault detection system which eliminates mechanical blocking means and which may be operated continuously rather than on an intermittent basis. Such a fault detection system will provide the inherent reliability necessary to ensure improved safety during the combustion of fossil fuel.
It is therefore an object of the present invention to provide a scanner fault detection apparatus and method which is capable of continuously monitoring the integrity of the scanner sensing apparatus and procedure and is capable of doing so without relying upon mechanical blocking means.